1. Field of the Invention
The invention relates to electrical resistors and more particularly to large power resistors employing a resistance element wound in the shape of a helix.
2. Description of the Prior Art
Large power resistors are widely used in the grounding of the neutral lead of three phase power systems, dynamic breaking, wound rotor motor control, and other industrial applications. It is desirable to attain a high volume efficiency, that is, a large mass of resistance material per unit volume. One way of accomplishing this is to configure the current-carrying portion of the resistor as an edge-wound helix of metal ribbon. For many popular ratings the helix does not have sufficient mechanical stability to be self-supporting and requires a support structure. Some prior art resistors employed a three point support structure inside the helix. Three U-channels supported toothed insulators, the insulators being fixed to the channels with an asbestos strip impregnated with a supersaturated sodium silicate solution. The channels were fixed and spaced by internal circular supports welded to the channels. After construction of the support assembly, the previously wound helix was screwed onto the support assembly. This configuration provided good heat dissipation because of the wide area for incoming cooler air at the bottom of the helix. However, the fixed internal support was expensive to manufacture due to the high cost of the many welding operations on the internal support. Since the helix was wound onto the support assembly after construction relatively large clearances were required, producing a loose ribbon-to-insulator fit. In addition the use of the asbestos in the resistor presented a possible health hazard to assembly workers.
Another type of resistor employed an internal support formed from a single steel bar having a rectangular cross section. Insulators were fixed to the two narrow-dimensioned sides of the bar with asbestos impregnated with supersaturated sodium silicate solution. After construction of this subassembly the previously wound helix was screwed onto the support assembly, thereby providing a two-point support structure. The manufacturing cost of this resistor was lower than the previously described three-point support resistor. However, the same problem of a loose fitting helix was present due to the necessity to screw the helix onto the support structure after assembly of the support structure. The health hazard from use of the asbestos was also present. In addition the unit exhibited limited mechanical stability in response to forces applied perpendicular to the plane of the longer cross sectional dimension of the support bar. Increasingly, specifications for resistors include requirements for mechanical stability in all directions in order to resist seismic forces.
Another type of resistor employed a pair of rectangularly sectioned steel bars supporting toothed insulators. The bars were inserted into the helix, laterally spread apart to force the toothed insulators into contact with the interior of the helix, and spot welded to permanently maintain this position. This resistor had the advantage of low manufacturing cost and a tight fit between the insulators and the interior of the helix. However, the large toothed insulators required by a two-point support restricted air flow, resulting in poor heat dissipation and lower current rating. In addition, the problem of limited stability when subjected to forces perpendicular to the plane of the longer cross sectional support dimension remained. It is desirable to provide a helical wound power resistor having a three-point support for good heat dissipation and less restricted air flow, a tight ribbon-to-insulator fit, a minimum of welding operations on the internal support prior to assembly the resistor, elimination of asbestos and adhesive material, good mechanical resistance to forces applied in all directions, and a low manufacturing cost.